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Iron is one of the most abundant elements in the Universe, along with lighter elements like hydrogen, oxygen, and carbon. Out in interstellar space, there should be abundant quantities of iron in its gaseous form. So why, when astrophysicist look out into space, do they see so little of it?
First of all, there’s a reason that iron is so plentiful, and it’s related to a thing in astrophysics called the iron peak.
Universe - Hydrogen - Helium - Nucleosynthesis - Stars
In our Universe, elements other than hydrogen and helium are created by nucleosynthesis in stars. (Hydrogen, helium, and some lithium and beryllium were created in Big Bang nucleosynthesis.) But the elements aren’t created in equal amounts. There’s an image that helps show this.
The reason for the iron peak has to do with the energy required for nuclear fusion and for nuclear fission.
Elements - Iron - Left - Fusion - Releases
For the elements lighter than iron, on its left, fusion releases energy and fission consumes it. For elements heavier than iron, on its right, the reverse is true: its fusion that consumes energy, and fission that releases it. It’s because of what’s called binding energy in atomic physics.
That makes sense if you think of stars and atomic energy. We use fission to generate energy in nuclear power plants with uranium, which is much heavier than iron. Stars create energy with fusion, using hydrogen, which is much lighter than iron.
Life - Star - Iron - Nucleosynthesis - Elements
In the ordinary life of a star, elements up to and including iron are created by nucleosynthesis. If you want elements heavier than iron, you have to wait for a supernova to happen, and for the resulting supernova nucleosynthesis. Since supernovae are rare, the heavier elements are rarer than the light elements.
It’s possible to spend an extraordinary amount of time going down the nuclear physics rabbit hole, and if you do, you’ll encounter an enormous amount of detail. But basically,...
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